ASTM D6272

What the test does

A rectangular bar of plastic or insulating board rests unclamped on two parallel rollers. Two further rollers descend onto its upper face, each set an equal distance from its nearest support, and bend the bar. Force and mid-span deflection are recorded continuously until the bar ruptures or reaches 5.0 % outer-fibre strain, whichever comes first.

What it measures, and why it matters

The method reports flexural strength — the modulus of rupture — flexural stress at 5.0 % strain for bars that never break, and flexural modulus. Modulus is a direct design input for the stiffness of a moulded rib, an enclosure wall or a laminate panel, and feeds finite-element models. Strength figures drive lot release and grade selection, and qualify electrical insulating boards. Because two noses spread peak stress across a length of material rather than concentrating it under one point, the result reflects the weakest section within that region.

Specimen

A bar of uniform rectangular cross-section, moulded or machined from sheet and tested flatwise across the supports. Geometry follows from the span-to-depth ratio: 16:1 is standard, 32:1 or 40:1 for high-strength reinforced composites, and 60:1 is recommended where modulus is wanted with minimal shear contribution. For the common 3.2 mm bar at 16:1 the support span is 51.2 mm and the load span is either 17.1 mm (one-third) or 25.6 mm (one-half). Conditioning follows Practice D618 — 23 ±2 °C and 50 ±10 % relative humidity — unless the material specification says otherwise. Replicate counts and formal rejection criteria belong to the standard's current text; in practice a bar that rocks on the supports, carries a visible flaw between the noses, or twists under load is replaced rather than reported.

What the machine must be capable of

Four-point loading splits the force between two noses, so reaching the same outer-fibre stress takes roughly twice the total force of a three-point test on the same bar. Expect tens of newtons for soft unreinforced grades and up to about 2–3 kN for stiff reinforced bars, so resolution at the low end matters more than headline capacity. The force system must be verified to ASTM E4.

Rate is fixed by procedure: Procedure A uses a strain rate of 0.01 mm/mm/min for small deflections and is preferred for modulus, Procedure B uses 0.10 mm/mm/min for large deflections and strength. Crosshead speed follows the standard's formula — for a one-third load span, R = 0.185·Z·L²/d — giving roughly 1.5 mm/min and 15 mm/min respectively for the 3.2 mm bar on a 51.2 mm span. Plastics are rate-sensitive, so the procedure used must be reported alongside the number.

Deflection or strain must be captured cleanly across 0–5 % outer-fibre strain; ASTM E83 is the referenced classification, with the required class set by the standard's text. The fixture is two supports and two loading noses, all parallel hardened cylinders — commonly 4, 6 or 10 mm diameter, seated in vee-grooves so they rotate rather than drag. Rollers that cannot turn add friction to the measured load. Testing away from the standard atmosphere needs an environmental chamber.

What goes wrong in practice

If the two noses are not coplanar, one contacts first and the bar sees a three-point condition until it seats, softening the initial slope and depressing modulus. A bar with non-parallel faces twists on the supports and loads a corner, scattering strength. Ductile grades often never rupture: the run ends at 5.0 % strain, and the figure reported is stress at 5.0 % strain, not strength. Swapping one-third and one-half load-span rollers without changing the stress equation puts every result out by a fixed factor.

Related and equivalent standards

ASTM D790 covers the same materials in three-point bending; the two are not interchangeable, and four-point commonly returns lower strength because more material is exposed to peak stress. ASTM D7264 addresses polymer matrix composites and offers both loading geometries. ISO 14125 is the nearest counterpart for fibre-reinforced plastics, with different specimens and spans, so results do not transfer. Practice D618, ASTM E4 and ASTM E83 supply conditioning, force verification and strain-instrument classification.